1. Field of the Invention
The present invention relates to insulated gate thyristor devices and, more particularly, to a turn-on/off driving technique for an insulated gate thyristor.
2. Description of the related art
Thyristors are semiconductor switching elements which can be shifted from a nonconductive state in which current does not flow through a thyristor device to a conductive state in which a current flows (conductive state) in response to a gate control signal applied to its gate electrode. In order to improve the switching characteristics of the thyristors, it is important to decrease energy loss of the thyristors at switching time and increase the maximum capability of the turn-on/off current. The improvement in the turn-on capability depends upon the allowable limit of the rising rate of a turn-on current (i.e., the rising rate of a critical current, known as "di/dt rating"). When a thyristor is driven to turn on, if an abrupt turn-on current exceeding the di/dt rating flows into the thyristor, a local current concentration occurs in its turn-on region. This leads to the breakdown of the device. Improvements in the turn-off capability of the device are desired fundamentally to prevent the turn-off current from locally concentrating and to stop substantially uniformly the turn-off current over the entire turn-off region of the device.
In order to improve di/dt rating of thyristors, a thyristor having a MOS gate structure is known, for instance. Its basic structure is disclosed, for example, in "A FET-controlled Thyristor in SIPOS Technology", IEDM, 1980 at p. 79. According to the thyristor disclosed therein, a metal oxide semiconductor (MOS) transistor is added and functions as a switching transistor for controlling to drive the turn-on of the thyristor. The MOS transistor has a part of the surface region of a second base layer as a channel region, a second emitter layer as a source, and a first base layer as a drain. It is reported that the "di/dt rating" of this thyristor was remarkably improved up to approx. 4000 amperes/microsecond.
Recently, more requirements have been requested for thyristors: the utility of the thyristors as switching elements for processing larger currents requires much larger di/dt rating for the thyristors. Furthermore, improvements in switching characteristics of the thyristors are strongly desired in the high frequency range. According to conventional thyristors, if a first base layer is enlarged in size so as to increase di/dt rating, energy loss at the turn-on time thereof is increased to thereby cause the high frequency characteristics of the thyristor to be deteriorated.
A MOS gate turn-off thyristor (also known as "MOSGTO thyristor") improved in its turn-off capability by adding an insulated gate as a second gate for driving the turn-off is disclosed, for example, in "MOS GTO--a Turn-off Thyristor with MOS-controlled Emitter Shorts", IEDM, 1985 at page 158. According to this thyristor, a second gate electrode for controlling the turn-off of the thyristor is insulatively provided above a second base layer. When the turn-off voltage (here, positive voltage) is applied to the second gate electrode, a second emitter layer is electrically shorted to the second base layer through a second channel region, which is disposed directly under the second gate in the second base layer, thus turning off the GTO thyristor. In such a structure, however, only one of first and second channel regions defined in the thyristor, i.e., only the second channel region, contributes to drive the turn-off of the thyristor. Therefore, a shorting resistance increases twice as large as the turn-off drive of the thyristor in both channel regions, and a peak turn-off current is reduced to approx. in half. This means that the turn-off capability of the thyristor is poor and its turn-off switching speed is improper.
A MOS gate turn-off thyristor (also known as "MOS controlled thyristor") in which a MOSFET is contained to control the turn-off is disclosed, for example, in "MOS Controlled Thyristors (MCT's)", IEDM, 1984 at page 282. According to this conventional thyristor, turn-off channel regions are defined at both sides of a second emitter layer. However, the turn-off channel region in contact with the turn-on channel region almost does not contribute to the turn-off of the thyristor. This occurs because that surface of the second base of the thyristor in which the turn-off channel region and the turn-on channel region are contacted with one another is set to a relatively high resistance, thus disabling turn-on current. Therefore, the thyristor of this type cannot expect rapid turn-on current and uniform cessation, thus reducing the turn-off capacity.